Galaxy clusters are the biggest celestial objects in the sky consisting of thousands of galaxies. They form from nonuniformity in the matter distribution established by cosmic inflation in the beginning of the Universe. Their growth is a constant fight between the gathering of dark matter by gravity and the accelerated expansion of the universe due to dark energy. By studying galaxy clusters, researchers can learn more about these biggest and most mysterious building blocks of the Universe.

Led by Hironao Miyatake, currently at NASA's Jet Propulsion Laboratory, Surhud More and Masahiro Takada of the Kavli Institute for the Physics and Mathematics (Kavli IPMU), the research team challenged the conventional idea that the connection between galaxy clusters and the surrounding dark matter environment is solely characterized by their mass. Based on the nature of the non-uniform matter distribution established by cosmic inflation, it was theoretically predicted that other factors should affect the connection. However, no one had succeeded in seeing it in the real Universe until now.

The Chandra image of Abell 1689 shown above is one of the most massive galaxy clusters known. The gravity of its trillion stars, plus dark matter, acts like a 2-million-light-year-wide "lens" in space that bends and magnifies the light of galaxies far behind it.

The image below shows the halo surrounding the Perseus Cluster, a swarm of galaxies approximately 250 million light years from Earth. Imagine a cloud of gas in which each atom is a whole galaxy—that's a bit what the Perseus cluster is like. It is one of the most massive known objects in the Universe.

The team divided almost 9000 galaxy clusters from the Sloan Digital Sky Survey DR8 galaxy catalog into two samples based on the spatial distribution of galaxies inside each cluster. By using gravitational lensing they confirmed the two samples have similar masses, but they found that the distribution of clusters was different. Galaxy clusters in which member galaxies bunched up towards the center were less clumpy than clusters in which member galaxies were more spread out. . The difference in distribution is a result of the different dark matter environment in which they form.

Researchers say their findings show that the connection between a galaxy cluster and surrounding dark matter is not characterized solely by the mass of clusters, but also by their formation history.